The invention relates to a magnetooptic recording medium and its manufacturing method and, more particularly, to a magnetooptic recording medium and its manufacturing method which are suitable to be applied to a magnetooptic disk, a magnetooptic tape, and a magnetooptic card in which Magnetically Induced Super Resolution reproduction can be performed.
Hitherto, in association with the realization of a high density of recording marks in a magnetooptic disk, a further increase in recording capacity is progressing. To increase the recording capacity as mentioned above, a method of realizing the high density by reducing a length of recording mark, narrowing a track pitch, and microminiaturizing a recording pit is used.
As means which is effective in realizing the high density, a Magnetically Induced Super Resolution technique (MSR) by which the recording mark smaller than a spot diameter of a laser beam can be reproduced has been proposed (for example, JP-A-2000-200448). According to the MSR, in the case where a main magnetic laminate film comprises a first magnetic layer, a second magnetic layer, and a third magnetic layer, by allowing the second magnetic layer to have inplane magnetization at room temperature in the spot of the laser beam, the magnetization in a low temperature portion is directed to an initialization magnetic field direction, thereby shutting off transfer of an information signal recorded in the third magnetic layer. In a middle temperature portion, since perpendicular magnetization is performed, the transfer of the signal is assisted by a switched connection force of the magnetic layers. Further, in a high temperature portion, by extinguishing the magnetization at a Curie temperature Tc, the transfer to the first magnetic layer is shut off, thereby enabling the signal smaller than the spot diameter of the laser beam to be reproduced.
Specifically speaking, as shown in FIG. 1, in a magnetooptic disk 100, assuming that the first magnetic layer is a reproducing layer 101, the second magnetic layer is an intermediate layer 102, and the third magnetic layer is a recording layer 103, when the magnetooptic disk 100 is rotated and the laser beam for reproduction is irradiated onto the magnetic laminate film, temperature distribution is caused in the magnetooptic disk 100. An arrow in FIG. 1 indicates a magnetizing state of the magnetooptic disk 100 upon reproduction.
In a state where the temperature distribution has been caused, in a low temperature area, when a reproducing magnetic field is larger than the switched connection force which acts on a portion between the intermediate layer 102 and the recording layer 103, the magnetizing direction of the intermediate layer 102 is aligned to the same direction as that of a reproducing magnetic field. The magnetizing direction of the reproducing layer 101 which has been switched-connected to the intermediate layer 102 is aligned to the direction opposite to that of the reproducing magnetic field irrespective of the recording mark. Thus, a front mask is formed. In a high temperature area, the switched connection force which acts on a portion between the reproducing layer 101 and the intermediate layer 102 is shut off and the magnetizing direction of the reproducing layer 101 is aligned to the same direction as that of the reproducing magnetic field, so that a rear mask is formed. In a middle temperature area, the switched connection force larger than the reproducing magnetic field acts on the portions between each of the reproducing layer 101, the intermediate layer 102, and the recording layer 103, so that the magnetizing direction of the recording layer 103 is transferred to the reproducing layer 101.
As mentioned above, when a magnetooptic output of the magnetooptic disk 100 is detected, the mask is formed in the low temperature area and the high temperature area in the spot of the laser beam. Therefore, a magnetooptic signal can be reproduced only from the middle temperature area without reproducing a magnetic signal of the area where the mask has been formed.
However, to realize microminiaturization of the recording mark as mentioned above, a sudden decrease in amplitude of the signal to be read out becomes a problem. Therefore, in the magnetooptic disk, to realize the even higher density in the future, improvement in magnetic resolution in the signal detection is necessary.
There is a variation in intensity of the laser beam which is irradiated from an optical pickup. Further, since the Magnetically Induced Super Resolution operation (MSR operation) is very sensitive to the intensity of the laser beam, not only the improvement in the magnetic resolution but also suppression of narrowing of a margin against a recording power are necessary.
Therefore, there has been strongly demanded a development of a technique of a magnetooptic recording medium such that even if the recording mark is microminiaturized and the track pitch is narrowed, the margin against the recording power is not narrowed but the magnetic resolution is improved and optical characteristics upon reproduction are satisfied.
It is, therefore, an object of the invention to provide a magnetooptic recording medium and its manufacturing method having high reliability such that a power margin of a laser beam can be assured in a wide range where recording and/or reproduction can be performed, so that an information signal recorded on the magnetooptic recording medium on which a recording mark length is microminiaturized and a track pitch is narrowed is reproduced while keeping good signal characteristics, high recording density can be realized, and a large capacity can be realized.